1. Field of the Invention
The present invention relates to a soldering flux, soldering paste, an electronic component device, an electronic circuit module, an electronic circuit apparatus and a soldering method.
2. Description of the Prior Art
A flux is used to solder a component onto a component mounting board in the known art. The main function of the flux is to improve the wettability of the solder by removing the oxide film formed at a metal conductor provided at the component mounting board and also at the surface of the metal to be soldered at the component. Among fluxes used in the related art, those containing rosin as the primary constituent are the best known. Rosin contains a carboxylic acid such as abietic acid or levopimaric acid, and the carboxyl group acts to remove the oxide film at the surface of the metal to be soldered.
Various additives such as solvents, plasticizers and thixotropic agents are normally mixed into a flux to improve the printability and increase the retentive strength. For instance, Japanese Unexamined Patent Publication No. 1999-121915 discloses a flux, the viscosity of which is adjusted by adding alcohol.
Other types of fluxes include the RMA (halogen-free) type fluxes conforming to the MIL Standard. This type of flux eliminates the necessity of washing the flux and the like after the reflow process.
Since none of the fluxes described above are used to bond the soldered component following the soldering process, the solder joint is achieved through fusion joining of the metal to be soldered. Accordingly, the degree of strength with which the metals to be soldered are bonded with each other is determined by the size of the solder joint area.
As components become further miniaturized to keep pace with higher density mounting pursued in various types of electronics devices, the pitch representing the distance between the individual components is becoming smaller, which, in turn, has resulted in a drastically reduced solder joint area. Thus, it is difficult to ensure a sufficient soldering strength even today. Since it is expected that higher density at the mounting board, further miniaturization of components and a smaller pitch between individual components will be pursued with increasing vigor in the future, it will become even more difficult to support this technical trend with the means for assuring the bonding strength in the related art that achieves the required bonding strength simply by allowing the solder joint to be formed over a large enough area.
The required solder joint strength is normally assured by forming a solder fillet portion and thus, increasing the solder joint area where a terminal of the component and a conductor (a land or a solder bump) on the component mounting board become soldered to each other. However, since the bonding area of the fillet portion itself is bound to be reduced in high-density mounting, the fillet portion cannot readily contribute to an increase in the bonding strength.
In addition, when manufacturing various types of electronic circuit modules, for instance, a component mounting board that allows components to be mounted on both sides is often utilized so that after soldering components in a furnace, onto one surface of the component mounting board with a high-temperature solder, components are mounted onto the other surface and the board undergoes a soldering process in the furnace again. This means that when soldering the components onto the other surface of the component mounting board, it is necessary to use a low-temperature solder having a lower melting point than the high-temperature solder used on the first side. In the prior art, the melting point of a solder is usually adjusted in conformance to Pb content.
At the same time, in response to the demand for solders that do not contain Pb (hereinafter referred to Pb-free solders) by the advocates of global environment conservation, great effort has been invested in at developing solders with Pb-free composition in recent years. However, no Pb-free solder composition with a high melting point comparable to that of a high-temperature solder in the prior art has been put into practical use to date. The main obstacle is that the melting point of Pb-free solder itself is approximately 220° C. that is higher than the melting point of a eutectic solder by as much as 40° C. and thus, no effective alternative to Pb has been discovered. Consequently, the issue of insufficient difference between the melting points of the solders used on the two sides of a double-sided installation type component mounting board is yet to be addressed, and the problem of components becoming lifted off or falling off when they are mounted onto the component mounting board is still common.
Furthermore, when manufacturing an electronic circuit module using semiconductor chips, an additional work process is implemented after the semiconductor chips are soldered onto the chip mounting board. A sealant is applied onto the bonding interface to firmly bond the semiconductor chips to the chip mounting board with the sealant.
However, if there is any residual flux present when the sealant is injected, the flux prevents the sealant from reaching the interface between the semiconductor chips and the board in sufficient quantity and thus, the sealant fails to achieve its full bonding strength. Accordingly, a process of washing away the flux is added before injecting the sealant. Under normal circumstances, the flux washing process is implemented by using a volatile organic solvent in a plurality of separate steps. However, stricter regulations are imposed today on the use of volatile organic solvents for environmental protection, and thus, the flux washing process has become an onerous process from the viewpoints of both cost effectiveness and environmental protection.
Moreover, when a combination of a ceramic substrate and an organic resin substrate is adopted to constitute the chip mounting board and the motherboard respectively to manufacture an electronic circuit apparatus by mounting an electronic circuit module obtained through the process described above onto the motherboard, the contraction stress tends to concentrate in the solder joint to result in cracks to occur readily in the solder joint to result in a reduction in the durability of the joints due to the difference between the coefficients of thermal expansion of the ceramic board and the organic resin board, as proven in thermal shock tests and the like conducted after the electronic circuit module is mounted. While it would be desirable to inject a sealant in order to improve the strength of the solder joint for this reason, it would lead to a considerable increase in the production cost to implement a flux washing process and a sealant injection process on the entire motherboard, and thus, the sealant is not injected in reality.